Formed core sandwich structure and method and system for making same

ABSTRACT

A method and apparatus for producing a metal core sandwich structure that is lightweight and many times stiffer than regular sheet metal, and which is easily formable into curved structures as well as structures having compound curves. In one embodiment, a formed metal core includes a plurality of cells comprising alternating front and rear projections extending outwardly in front of and behind a median plane, with each projection having a bonding surface area or land configured to be brazed or bonded with corresponding external metal sheets on both sides of the formed metal core. A plurality of micro-abrasions or indentations are formed on the bonding lands, allowing stronger brazing or bonding joints to be formed between the metal core and the external metal sheets by facilitating improved capillary action by the metal core during the brazing or bonding process.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. applicationSer. No. 11/285,096, filed on Nov. 22, 2005, entitled “Formed Metal CoreSandwich Structure and Method and System for Making Same” and thedisclosures of which are herein incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to metal sandwich sheet structures and,more particularly, to formable metal core sandwich sheet structures andmethods and systems for manufacturing same.

2. Description of Related Art

Metal core sandwich structures are known in the art. For example, U.S.Pat. No. 3,525,663 to Hale and U.S. Pat. No. 4,035,536 to Morrison bothdisclose metal sandwich structures having a formed metal core brazed totwo outer metal sheets. However, it is believed that the structuresdisclosed by Hale and Morrison, and other similar conventional formedmetal core sandwich structures, cannot be easily formed into compoundcurves and other complex shapes without delaminating, wrinkling and/orotherwise losing its structural integrity. Therefore, prior metal coresandwich structures are not well-suited for applications that requireformation of compound curves, for example, without delaminating orwrinkling during the forming process.

Other types of metal core sandwich structures, such as honeycombsandwich structures, are described in U.S. Pat. No. 6,544,623 to Straza,for example. Although this honeycomb core sandwich structure providesimproved formability when compared to other metal core sandwichstructures, it is relatively expensive to manufacture and, therefore,not well-suited for many commercial, cost-sensitive applications (e.g.,automobile bodies).

Therefore, there is a need for an improved metal core sandwich structurethat is more economical to manufacture and which exhibits improvedformability characteristics without sacrificing structural integrity andstrength.

BRIEF SUMMARY OF THE INVENTION

The invention addresses the above and other needs by providing animproved metal sandwich sheet structure that includes a formed metalcore that is brazed or bonded to two outer flat metal sheets. Thisimproved metal sandwich structure is capable of being formed intocomplex shapes while retaining its structural integrity. This improvedmetal sandwich structure is also easier and more economical tomanufacture than prior honeycomb core sandwich structures, whileproviding similar strengths and characteristics as the honeycomb corestructures.

In one embodiment of the invention, a method of producing a metal coresandwich structure includes providing a formed metal core that islightweight and many times stiffer than regular sheet metal, and whichis easily formable into curved structures as well as structures havingcompound curves that curve in two or more directions simultaneously, forexample. In one embodiment, a metal core sheet is rolled, stamped orpressed into a corrugated shape having a plurality of cells comprisingalternating front and rear projections extending outwardly in front ofand behind a median plane, with each projection having a bonding surfacearea or land configured to be brazed or bonded with correspondingexternal metal sheets on both sides of the formed metal core sheet. Aplurality of micro-abrasions or indentations are formed on the bondinglands, allowing stronger brazing joints to be formed between the metalcore and the external metal sheets by facilitating improved capillaryaction by the metal core during the brazing process. The improvedbrazing joints provide better bonds so the resulting multi-layer sheetstructure will not fracture or delaminate during forming. The resultingsandwich structure is easier to form into complex curves and shapeswhile maintaining its structural integrity. Thus, the improved metalsandwich structure is well suited for various applications whereformability and structural integrity are important (e.g., automotivebody applications).

In another embodiment of the invention, an apparatus for forming animproved core metal sheet is provided. The apparatus includes a press ormold that presses (via stretching and/or gathering) the core metal sheetto form a desired corrugated core structure. At desired locations on thesurface of the press, micro-protrusions and/or micro-ridges and/ormicro-lines (collectively referred to herein as “micro-protrusions”)extend outwardly from the press surfaces so that during forming of thecore metal sheet, micro-abrasions and/or micro-grooves and/ormicro-channels (collectively referred to herein as “micro-abrasions”)are formed to extend inwardly on desired surface areas of the formedmetal core. These micro-abrasions allow for improved brazing between theformed metal core and the two outer metal sheets through improvedcapillary action by the metal core during brazing.

In a further embodiment, the outer layer sheets are electro-tacked tothe formed core prior to brazing or bonding, which allows for improvedsubsequent brazing and an improved quality sandwich structure. In oneembodiment, the invention provides a novel electro-tacking apparatus fortack welding the formed core to the outer metal sheets prior to brazing.

In various embodiments of the invention, various cell shapes andgeometries such as hexagonal, square, octagonal or other shape may beformed in the core sheet as well as different cell sizes in accordancewith desired formability, stiffness and crush characteristics. Forexample, it may be desirable for some zones of the sandwich structure tobe more easily crushed (e.g., to provide “crush zones” in an automobilebody frame) or to be more rigid or more flexible. For example, if crushzones are desired, the sandwich structure (or at least designated areasof the sandwich structure) can be designed to be more easily crushedwhich may help avoid automobile impact injuries while other areas of theautomobile body (e.g., roofs) will be stiffer, allowing for moreprotection from metal crushing into passenger compartments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of a metal core sheet after it hasbeen formed, in accordance with one embodiment of the invention.

FIG. 2A illustrates a top view of the formed metal core sheet of FIG. 1.

FIG. 2B illustrates a cross-sectional side view of the formed metalsheet of FIGS. 1 and 2A, taken along a cross-section indicated by lines2B-2B of FIG. 2A.

FIG. 3A illustrates a top view of a metal core sheet formed in acorrugated checker-board pattern, in accordance with another embodimentof the invention.

FIG. 3B illustrates a cross-section side view of the formed metal coreof FIG. 3A taken along lines 3B-3B of FIG. 3A.

FIG. 4 illustrates a side view of a metal core sheet being formedbetween two press plates or molds, in accordance with one embodiment ofthe invention.

FIG. 5 illustrates an exploded side view of a metal core sandwichstructure prior to brazing, in accordance with one embodiment of theinvention.

FIG. 6 illustrates the metal core sandwich structure of FIG. 4 afterbrazing.

FIG. 7 illustrates a perspective view of an electro-tack weldingapparatus used to tack weld the metal core sandwich structure prior tobrazing, in accordance with a further embodiment of the invention.

FIG. 8 illustrates a cross-sectional side view of one embodiment of adouble cell sandwich structure.

FIG. 9 illustrates a top view of a press plate used to form a core sheetused in the embodiment of FIG. 8.

FIG. 10 is cross-sectional side of the embodiment of FIG. 8 showingexemplary dimension.

FIG. 11 illustrates a cross-sectional side view of a further embodimentof a double cell sandwich structure.

FIG. 12 a illustrates a top view of an upper press used to form a firstcore sheet used in the embodiment shown in FIG. 11.

FIG. 12 b is a cross-sectional side view of the upper press of FIG. 12and a lower press used for forming a first core sheet, in accordancewith one embodiment of the present invention.

FIG. 13 a illustrates top view a top view of an upper press used to forma second core sheet used in the embodiment shown in FIG. 11.

FIG. 13 b is a cross-sectional side view of the upper press of FIG. 13a, taken along the lines 13 b-13 b of that figure, and a lower press forforming a second core sheet, in accordance with one embodiment of thepresent invention.

FIG. 14 illustrates a cross-sectional side view of an exemplaryembodiment of a double cell sandwich structure having a middle sheet, inaccordance with one embodiment of the present invention.

FIG. 15 illustrates a cross-sectional side view of another exemplaryembodiment of a double cell sandwich structure having a middle sheet, inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the invention are described in detail belowwith reference to the figures wherein like elements are referenced withlike numerals throughout. It is understood that the figures are notnecessarily drawn to scale but intended to merely illustrate some of thenovel aspects, features and processes of the invention.

FIG. 1 illustrates a perspective view of a formed metal core 10 made inaccordance with one embodiment of the invention. In this embodiment, themetal core 10 is formed in a corrugated pattern having a plurality ofcells 12 comprising alternating front and rear projections extendingoutwardly in front of and behind a median plane 14, with each projectionhaving a bonding surface area or land 16 configured to be brazed orbonded with corresponding external metal sheets (not shown) on bothsides of the median plane 14. As shown in FIG. 1, a plurality ofmicro-abrasions or indentations 18 are formed on the bonding lands 16.As explained in further detail below, the micro-abrasions 18 providestronger brazing joints to be formed between the metal core 10 and theexternal metal sheets by facilitating improved capillary action by themetal core 10 during the brazing process.

FIG. 2A illustrates a top view of the formed metal core 10 of FIG. 1.The metal core 10 includes a plurality of upwardly projecting cells 12Aand a plurality of downwardly projecting cells 12B. The bonding lands 16of the upwardly projecting cells 12A are also shown while the bondinglands 16 of the downwardly projecting cells 12B are located on theopposite side of the core 10 and, hence, not shown in FIG. 2A. Aplurality of micro abrasions or indentions 18 are formed on the bondinglands 16 of each cell 12A and 12B to facilitate improved capillaryaction of the metal core 10 during brazing.

FIG. 2B illustrates a cross-sectional side view of the metal core 10taken along perspective lines 2B-2B of FIG. 2A. As shown in FIGS. 1, 2Aand 2B, in one embodiment, each cell 12A and 12B is formed in the shapeof an octagon. However, it is understood that the invention is notlimited to any particular shape, size and/or configuration of the cells12A, 12B. Any desired shape, size and configuration of cells may beutilized to achieve various desired physical characteristics of theresulting metal core sandwich structure, in accordance with the presentinvention.

FIG. 3A illustrates a top view of an alternative, exemplary formed metalcore 30 having a plurality of square-shaped cells 32A and 32B in acheckerboard configuration. The cells 32A represent upwardly projectingcells and the cells 32B represent downwardly projecting cells. Each cell32A and 32B has a corresponding bonding land 34 and a plurality ofmicro-abrasions 36 on the surface of each bonding land 34. FIG. 3Billustrates a cross-sectional side view of the formed metal core 30 ofFIG. 3A, taken along lines 3B-3B of that figure.

FIG. 4 illustrates a cross-sectional side view of the metal core 30 asit is formed between two presses 42 and 44 having corresponding moldedpatterns and configurations to form the metal core 30 into a desiredshape and configuration. During forming, a flat sheet of metal material30 is placed between the top and bottom presses 42 and 44, respectively.As the top and bottom presses 42 and 44 are pressed together the metalcore 30 is formed via stretching and/or gathering of the metal sheetmaterial 30 to form the cells 32A and 32B in a checkerboardconfiguration. It is understood that the presses 42 and 44 are exemplaryonly and that presses having other desired molded patterns andconfigurations may be utilized to form a metal core having cells ofvarious shapes (e.g., octagons, hexagons, pentagons, squares, triangles,etc.), and sizes, and configurations.

In one embodiment, the micro-abrasions 36 illustrated in FIG. 3A, forexample, are formed during pressing of the metal core 30. A plurality ofmicro-protrusions, ridges and/or lines (collectively referred to hereinas “micro-protrusions”) (not shown) are provided on selected surfaces 46and 48 of top and bottom presses 42 and 44, respectively. Duringpressing of the metal core 30, the micro-protrusions form the pluralityof micro-abrasions 36 on the bonding lands 34 of the cells 32A, 32B ofthe formed metal core 30. In one embodiment, the micro-protrusions areformed on the surfaces of the presses 42 and 44 corresponding to thebonding lands 34 of the formed metal core 30. In one embodiment, themicro-protrusions are formed on the corresponding press surfaces by fineetching of press surfaces using known means and techniques (e.g.,electrical metal etching, chemical etching, sand blasting, gritblasting, etc.). In one embodiment, the micro-protrusions are formed soas to provide micro-abrasions 36 having a depth and/or width of 0.0005inches to 0.002 inches and cover approximately 50-80% of the surfacearea of the bonding lands 34. In various embodiments, the abrasions 36may be substantially parallel channels, criss-cross configurations, orchannels traveling in random directions and/or curved channels. Inalternative embodiments, the micro-abrasions may be formed by electronicdischarge machining (EDM) processes, which are well-known in the art. Inother embodiments, the micro-abrasions 36 may be formed on the bondinglands 34 of the metal core 30 after the metal core 30 has been pressedand formed. It is appreciated, however, that providing presses 42 and 44with preformed micro-protrusions that automatically form micro-abrasions36 on the metal core 30 during pressing can save substantial time andwork when compared to forming the micro-abrasions 36 manually orindividually for each metal core 30 subsequent to pressing.

In alternative embodiments, the metal core 10, 30 may be formed after abrazing material has been placed on each side of the core 10, 30. Thus,the metal core sheet, plus brazing material on each side of the metalcore sheets, are pressed and formed together by the presses 42 and 44.The brazing material can be placed onto the surface of the metal core10, 30 prior to forming the core 10, 30 by spraying a brazing powderonto the core sheet to be formed or by placing sheets of brazingmaterial 50 on either side of the core sheet to be formed. With theproper amount of pressure during forming, the brazing material will beembedded into the core sheet. This facilitates the subsequent brazingprocess by evenly distributing the braze material across the surface ofthe core sheet to be formed. In these embodiments, the micro-abrasions18, 36 are formed and pass through the brazing material onto the bondinglands 16, 34 of the formed metal core 10, 30. Thereafter, the formedmetal core 10, 30 plus formed brazing material sheets are sandwichedbetween two external metal sheets and brazed at high temperature (e.g.,670 to 730 degrees Celsius) in a reduced-atmosphere or vacuum furnace toproduce a metal core sandwich structure in accordance with the presentinvention.

FIG. 5 illustrates an exploded cross-sectional side view of an exemplaryformed metal core 10 (FIG. 1) sandwiched between two sheets of brazingmaterial 50, which are in turn sandwiched between two external metalsheets 60. In this embodiment, the brazing material sheets 50 have notbeen pressed and formed with the metal core 10, as described above. FIG.6 illustrates a cross-section side view of the resulting metal coresandwich structure 70 after brazing of the metal core 10 to the externalmetal sheets 60 has been completed. The micro-abrasions 18 (FIG. 2A)allow for an improved capillary effect so that the brazing material 50,upon melting, flows into the pores of the bonding lands 16 (FIGS. 1 and2A) more easily and thereby provides a stronger braze joint between theclosely fitted surfaces of the bonding lands 16 and the external metalsheets 60. The improved capillary effect and, hence, improved brazingjoint provided by the micro-abrasions 18 produce a stronger metal coresandwich structure 70 that can be formed into compound curves, and othershapes and forms, more easily without delaminating, wrinkling orotherwise losing its structural integrity. In further embodiments,micro-abrasions may also be formed on the interior surface of theexternal metal sheets 60 in order to improve the capillary effect of theexternal metal sheets 60 during brazing.

In a further embodiment of the invention, prior to brazing, all of thelayers of the metal sandwich structure 70, comprising a formed metalcore 10, brazing material sheets 50 and two outer metal sheets 60, areelectro-tack-welded together so as to secure the multiple layers to oneanother and prevent relative movements during the brazing process. Thisfurther improves the strength of the braze joints formed between thebonding lands 16 of the core 10 and the external metal sheets 60 toprovide a higher quality finished product.

FIG. 7 illustrates a perspective view of a novel tack welding apparatus100 that may be used in conjunction with the present invention. Theapparatus 100 includes a computer and/or electronically controlledactuator box 102 that contains standard electronics and mechanicalcomponents (e.g., gears, switches, power supply, etc.) to controlmovement of an actuator arm 104 that controls movement of a top plate106. The metal core sandwich structure 70 (FIGS. 5 and 6) is positionedbetween the top plate 106 and a bottom ground plate 108. As shown inFIG. 7, the top tack-welding plate 106 comprises a plurality ofsubsections 110 that are electrically insulated from each other. In oneembodiment, the subsections 110 can be made from copper or copper-alloyand are insulated from one another by an insulating material (e.g.,ceramic). Each of the subsections 110 are electrically coupled to acorresponding conductive wire 112 via corresponding contacts 114. Theconductive wires 112 are coupled to a power switch box 116 that controlswhen electrical current is supplied to each individual wire 112. Thepower switch box 116 can be controlled by a computer or processor (notshown).

In one embodiment, a high energy, low voltage current is sequentiallyapplied to each subsection 110 to tack-weld a corresponding portion ofthe metal core sandwich structure 70 located between the correspondingsubsection 110 and the bottom ground plate 108. In one embodiment, thetop and bottom plates 106 and 108, respectively, apply an even pressurethroughout the sandwich structure 70. A sequence of tack welds arerapidly performed in different areas of the sandwich structure 70 bysequentially applying an electrical current to each subsection 110 ofthe top plate 106. By sequentially tack-welding only portions of thesandwich structure 70 at any one time, this tack-welding process allowsfor controlled heating and cooling throughout the sandwich structure 70,which in turn allows for improved tack welds. Thus, portions of thesandwich structure 70 following tack-welding are allowed to cool whileother portions are being tack-welded. This cooling process providesbetter tack-welds and the resulting sandwich structure 70 has a lowertendency to delaminate at individual tack welds. In other words, lettingthe structure 70 heat up and cool in different areas as it is being tackwelded helps prevent individual tack welds from separating. After themetal core sandwich structure 70 has been tack welded together, asdescribed above, the structure 70 is then placed in a furnace forbrazing.

The metal core sandwich structure 70 may incorporate any type of metaland/or metal alloys as the formed core 10, 30 and the external metalsheets 60, such as A1 or D2 steel, for example. It is understood thatany metals, metal alloys, or combinations thereof, that are suitable forbrazing are contemplated to be within the scope of the present invention(e.g., copper, aluminum, titanium, etc.). Additionally, any type ofknown brazing material in the form of sheets, foils, sprays , powders,paste or slurries, for example, may be utilized in accordance with thepresent invention. Furthermore, it is contemplated that some embodimentsof the invention may utilize non-metal core materials and non-metalexternal sheets. For example, known synthetic and/or polymer materials(e.g., Kevlar) may be used to form an injection-molded core andthereafter bonded (e.g., sonically welded and/or vibration welded) tosynthetic and/or polymer outer sheets. Micro-abrasions are formed on thebonding lands of the non-metal core which facilitate gripping betweenthe core and the external sheets during and after welding.

Further embodiments of metal core sandwich structures may include usinga double cell sandwich core. These embodiments also have two externalsheets defining a central core; but, instead of using a single coresheet (such as sheet 10 of FIG. 6), exemplary embodiments of double cellsandwich structures may use two core sheets.

One embodiment of a double cell sandwich structure 800 is shown in FIG.8. Double cell sandwich structure 800 can comprise external sheets 802and 804 sandwiching a first core sheet 806 a and a second core sheet 806b. External sheets 802 and 804 may be the same as sheets 60 shown inFIG. 6. First core sheet 806 a and second core sheet 806 b may be madeof the same materials as detailed above with reference to core sheet 10.In the embodiment 800, the first core sheet 806 a and the second coresheet 806 b comprise a plurality of pyramid-shaped cells 832 a, 832 b.

Further to FIG. 8, inwardly projecting lands 834 a of the first sheet806 a contact corresponding inwardly projecting lands 834 b of thesecond sheet 806 b. In addition, outwardly extending lands 836 a of thefirst sheet 806 a contact an interior side of the external sheet 802 andoutwardly extending lands 836 b of the second sheet 806 b contact aninterior side the external sheet 804. The inwardly projecting landings834 a of the first sheet can be configured to be brazed or bonded withcorresponding inwardly facing landings 834 b of the second sheet.Moreover, outwardly facing landings 836 a and 836 b of the first sheetand the second sheet, respectively, can be configured to be brazed orbonded with the interior surface of corresponding external metal sheets802 and 804. It is understood that any brazing or bonding techniquedescribed above may be similarly used with the lands 834 and 836 andexternal sheets 802 and 804 to facilitate gripping between the coresheets and the core sheets and external sheets. In one embodiment,micro-abrasions 18 are formed on a plurality of lands 834 and/or 836 toimprove capillary action during the brazing process.

FIG. 9 illustrates a top view of a press 900 configured to press or moldcore sheets 806 a and 806 b, in accordance with one embodiment of thepresent invention. Press 900 has corresponding molded patterns, such aspunch 902, and configurations to form the cores sheets 806 a and 806 b.During forming, a flat sheet of metal material can be placed between thetop press 900 and a corresponding bottom press (not shown). As the toppress 900 and the bottom press are pressed together, a core sheet isformed via stretching and/or gathering of the metal sheet. It isunderstood that press 900 is exemplary only and that presses havingother desired molded patterns and configurations may be used to formmetal core sheets having cells of various shapes, sizes andconfigurations.

FIG. 10 is a cross-sectional view of structure 800 showing exemplarydimensions of the structure 800. The exemplary dimensions are asfollows: an inwardly projecting landing thickness A=0.0040″; an insideradii B=0.0006″; an outside radii C=0.010″; a cell diameter D=0.050″; acell incline E=70°; an outwardly projecting landing thickness F=0.0040″;an external sheet thickness G=0.0060″; a core sheets width H=0.0300″;and a sandwich cell structure width I=0.0720″. It is understood that theabove dimensions are exemplary only and that a sandwich core structurehaving other sizes and configurations may be used as desired to fit theneeds of a particular application.

FIG. 11 is a cross-sectional side view of a further embodiment of adouble cell sandwich structure 1100. Structure 1100 has external sheets1102 and 1104 sandwiching first core sheet 1006 and second core sheet1108. The first core sheet 1006 has a generally sinusoidalcross-sectional shape with a bulge 1110 located at a tip of eachinwardly projecting peak 1112. The second core sheet 1108 also has agenerally sinusoidal cross-sectional shape, but has a dimple 1114located at a tip of each inwardly projecting peak 1116. As seen in FIG.11, the bulges 1110 sit in the corresponding dimples 1114.

The bulges 1110 and dimples 1114, as well as area near the bulges anddimples, may be brazed or bonded to facilitate gripping with acorresponding surface. Moreover, outwardly facing landings 1118 a and1118 b of the first sheet 1106 and the second sheet 1108, respectively,can be configured to be brazed or bonded with the interior surface ofcorresponding external metal sheets 1104 and 1102. As is appreciated,any suitable brazing or bonding technique described in this disclosuremay be used. In one embodiment, micro-abrasions 18 are formed onrespective surfaces to improve capillary action during the brazingprocess.

Similar to embodiments described above, the first core sheet 1106 andthe second core sheet 1108 may be formed between two presses havingcorresponding molded patterns and configurations to form a flat metalsheet into the desired shape and configuration.

FIG. 12 a is a top view of an upper press 1202 having a plurality ofpins 1206 for forming the first core sheet 1006. In one embodiment, pins1206 are spaced an equal distance away from each adjacent pin 1206. In afurther embodiment, partial pins that would otherwise be located atedges of the press 1202 are eliminated. This is believed to help avoidsharp edges which could cut the material being formed. Only five pins1206 are shown in FIG. 12 a for ease of understanding for the reader. Asis appreciated, there is no limit as to the number of pins 1206 that canbe used.

FIG. 12 b illustrates the cross-sectional side view of the upper press1202, taken along the lines 12 b-12 b of FIG. 12 a, and lower press 1204forming the first core sheet 1106 in accordance with one embodiment ofthe present invention. As can be seen, pins 1206 a, 1206 b, 1206 c havea generally conical shape and extend outwardly from a baseline 1208 ofthe upper press 1202. Located at a tip of each pin 1206 a, 1206 b, 1206c is a projection 1210. The lower press 1204 has a plurality ofconically-shaped cavities 1212 a, 1212 b, 1212 c, each cavity 1212having an indentation 1214 located at a central bottom area of eachcavity 1212. Pins 1206 a, 1206 b, 1206 c extends into respectivecavities 1212 a, 1212 b, 1212 c when the upper press 1202 and lowerpress 1204 are pressed together to form the first core sheet 1106.

FIG. 13 a is a top view of an upper press 1302 having a plurality ofpins 1306 for forming the second core sheet 1108. As with the upperpress 1202, pins 1306 may be spaced an equal distance away from eachadjacent pin 1306 and partial pins that would otherwise be located atedges of the press may be eliminated. Eliminating partial pins on theedges of a press is believed to help avoid sharp edges which could cutthe material being formed. Also, only five pins 1306 are shown in FIG.13 for ease of understanding for the reader. As is appreciated, however,there is no limit as to the number of pins 1306 that can be used.

FIG. 13 b illustrates a cross-sectional side view of upper press 1302,taken along the lines 13 b-13 b of FIG. 13 a, and lower press 1304forming a second core sheet 1108 in accordance with an embodiment of thepresent invention. The upper press 1302 and lower press 1304 can besimilar to the upper press 1202 and lower press 1204, respectively, withan exception being that that the upper press 1302 has indentations 1306instead of projections 1210 and the lower press 1304 has projections1308 instead of indentations 1214. In other words, the placement onrespective presses of the projections and indentations for forming thefirst core sheet 1106 is reversed for forming the second core sheet1108. Also, the indentations 1306 and projections 1308 are preferablylarger than the indentations 1214 and projections 1210 to permit thebulges 1110 formed in the first sheet 1106 to sit in the dimples 1114formed in the second sheet 1108, as shown in FIG. 11.

FIG. 14 illustrates a cross-sectional side view of a double cellsandwich structure 1600 having a middle sheet in accordance with anotherembodiment of the present invention. Some parts of the structure 1400are identical to those of the previously described embodiments, and likereference numerals have been used for like parts as appropriate. Asshown in FIG. 14, a middle sheet 1450 is positioned between the firstcore sheet 806 a and the second core sheet 806 b.

FIG. 15 illustrates a cross-sectional side view of a further embodimentof a double cell sandwich structure 1500 having a middle sheet, inaccordance with another embodiment of the present invention. Some partsof the structure 1500 are identical to those of the previously describedembodiments, and like reference numerals have been used for like partsas appropriate. As shown in FIG. 15, a dimpled middle sheet 1550 ispositioned between the first core sheet 1106 and the second core sheet1108. The dimpled middle sheet 1550 has dimples 1552 formed at locationscorresponding to the locations of the bulges 1110 of the first coresheet 1106 and the dimples 1114 of the second core sheet 1108.

It is understood that sheets forming the respective structures 1400 and1500 may be bonded or brazed together to facilitate gripping using thebonding or brazing techniques described in this disclosure.

The double cell sandwich structures provide many advantages. Forexample, it is believed that embodiments of double cell sandwichstructures provide many of the advantages of the single cell structures;yet double cell structures can provide even more strength and furtherreduce dimpling and wrinkling on the surface of the structure during andafter forming of the sandwich structure. Embodiments using of a middlesheet between the core sheets can strengthen the structure even further.

Various preferred embodiments of the invention have been describedabove. However, it is understood that these various embodiments areexemplary only and should not limit the scope of the invention asrecited in the claims below. Various modifications of the preferredembodiments described above can be implemented by those of ordinaryskill in the art, without undue experimentation. These variousmodifications are contemplated to be within the spirit and scope of theinvention as set forth in the claims below.

1. A method of manufacturing a metal core sandwich structure,comprising: forming a first metal core having a plurality of inwardlyand outwardly projecting bonding lands; forming a second metal corehaving a plurality of inwardly and outwardly projecting bonding lands;forming micro-abrasions on a plurality of the bonding lands of eachmetal core; placing the inwardly projecting bonding lands of the firstmetal core in contact with the inwardly projecting bonding lands of thesecond metal core; placing a first metal sheet in contact with theoutwardly projecting bonding lands of the first metal core and placing asecond metal sheet in contact with the outwardly projecting bondinglands of the second metal core, thereby forming a sandwich structure;and heating the sandwich structure to braze the first metal core, thefirst metal sheet, the second metal core and the second metal sheettogether, wherein the micro-abrasions allow for improved capillaryaction at the bonding lands.
 2. The method of claim 1, furthercomprising forming a bulge on each inwardly projecting land of the firstmetal core and forming a dimple on each inwardly projecting land of thesecond metal core, wherein the bulges are configured to be received atleast partially within corresponding dimples when the first and secondmetal cores contact each other.
 3. The method of claim 1 wherein thefirst and second metal cores are formed with a press.
 4. The method ofclaim 1 wherein first metal sheet and the second metal sheet comprise aplurality of cells, each of the cells having a pyramid-like shape. 5.The method of claim 1 wherein the micro-abrasions comprise grooves thatare approximately 0.0005 to 0.002 inches in depth.
 6. The method ofclaim 1 wherein the micro-abrasions cover approximately 50-80% of thesurface area of the bonding lands.
 7. The method of claim 1 wherein themicro-abrasions are formed on the bonding lands during the forming ofthe first metal core and second metal core.
 8. The structure of claim 1further comprising tack welding the first metal core, the second metalcore, the first metal sheet and the second metal sheet together.
 9. Ametal core sandwich structure comprising: first and second metal coreseach having a plurality of inwardly and outwardly projecting bondinglands, a plurality of the inwardly projecting lands of the first metalcore brazed to a plurality of the inwardly projecting lands of thesecond metal core; and a first metal sheet brazed to a plurality of theoutwardly projecting bonding lands of the first metal core and a secondmetal sheet brazed to a plurality of the outwardly projecting lands ofthe second metal core, wherein a plurality of micro-abrasions are formedon the plurality of bonding lands prior to brazing so as to allow forimproved capillary action at the bonding lands.
 10. The structure ofclaim 9 further comprising a bulge on each inwardly projecting land ofthe first metal core and a dimple on each inwardly projecting land ofthe second metal core, wherein the bulges are received at leastpartially within corresponding dimples.
 11. The structure of claim 9wherein the first metal core is tack welded to the second metal core andthe first metal sheet, and the second metal core is tack welded to thesecond metal sheet.
 12. The structure of claim 9 wherein the first andsecond metal cores each have a generally sinusoidal cross-sectionalshape.
 13. The structure of claim 9 wherein the first and second metalcores each comprise a plurality of pyramid-like shaped cells.
 14. Thestructure of claim 9 wherein the micro-abrasions comprise grooves thatare approximately 0.0005 to 0.002 inches in depth.
 15. The structure ofclaim 9 wherein the micro-abrasions cover approximately 50-80% of thesurface area of the bonding lands.
 16. The structure of claim 9 whereinthe micro-abrasions are formed on the bonding lands during forming ofthe respective first and second metal cores.
 17. A metal core sandwichstructure comprising: first and second metal cores each having aplurality of inwardly and outwardly projecting bonding lands; a firstmetal sheet brazed to a plurality of the outwardly projecting bondinglands of the first metal core; a second metal sheet brazed to aplurality of the outwardly projecting lands of the second metal core;and a third metal sheet brazed, on one side, to a plurality of theinwardly projecting lands of the first metal core and, on the otherside, brazed to a plurality of the inwardly projecting lands of thesecond metal core, wherein a plurality of micro-abrasions are formed ona plurality of the bonding lands prior to brazing so as to allow forimproved capillary action at the bonding lands.
 18. The structure ofclaim 17 further comprising a bulge on each inwardly projecting land ofthe first metal core and a dimple on each inwardly projecting land ofthe second metal core, wherein the bulges are received at leastpartially within corresponding dimples.
 19. The structure of claim 18wherein the third metal sheet comprises a plurality of bulges, at leastsome of the bulges of the third metal sheet receiving one of thecorresponding bulges of the first metal core and positioned in one ofthe corresponding dimples of the second metal core.
 20. The structure ofclaim 17 wherein the first and second metal cores each have a generallysinusoidal cross-sectional shape.
 21. The structure of claim 17 whereinthe first and second metal cores each comprise a plurality ofpyramid-like shaped cells.
 22. The structure of claim 17 wherein themicro-abrasions comprise grooves that are approximately 0.0005 to 0.002inches in depth.
 23. The structure of claim 17 wherein themicro-abrasions cover approximately 50-80% of the surface area of thebonding lands.
 24. The structure of claim 17 wherein the micro-abrasionsare formed on the bonding lands during forming of the first and secondmetal core.